Silicon-based electro-optic device

ABSTRACT

In an electro-optic device, a stack structure including a first silicon layer of a first conductivity type and a second silicon layer of a second conductivity type has a rib waveguide shape so as to form an optical confinement area, and a slab portion of a rib waveguide includes an area to which a metal electrode is connected. The slab portion in the area to which the metal electrode is connected is thicker than a surrounding slab portion. The area to which the metal electrode is connected is set so that a range of a distance from the rib waveguide to the area to which the metal electrode is connected is such that when the distance is changed, an effective refractive index of the rib waveguide in a zeroth-order mode does not change.

This application is based upon and claims the benefit of priority fromSingapore patent application No. 201001336-5, filed on Mar. 1, 2010, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electro-optic device formed on asilicon platform and utilized in the field of, for example, opticalcommunication, and in particular, to a silicon-based electro-opticdevice formed on an SOI (Silicon-On-Insulator) substrate.

2. Description of the Related Arts

Information communication networks typified by the Internet areconstructed so as to spread throughout the world as socialinfrastructure essential for peoples' lives. Optical communication usingoptical fibers is a technique to support huge traffics on the Internet.Optical communication devices using silicon platforms can utilize a1.3-μm band and a 1.55-μm band of wavelengths, which are included in thewavebands used for optical fiber communication. Furthermore, theseoptical communication devices can be manufactured by a CMOS(Complementary Metal-Oxide-Semiconductor) device fabricating processes.Thus, the optical communication devices are expected to implementhigh-density optical integrated circuits.

Increasing an information transmission rate per channel is a method fordealing with traffic on information communication networks, whichincrease year by year. An optical modulator is important forimplementing this method; the optical modulator quickly convertselectric signals from an LSI (Large-Scale Integration) circuit intooptical signals. The LSI circuit processes information in an opticalcommunication device. Thus, implementation of the optical modulator on asilicon platform has been proposed.

A typical proposed optical modulator is of a type that utilizes acarrier plasma effect to change the refractive index of a siliconmaterial and thus to change propagation characteristics of light. Forexample, A. Liu et al. have proposed a silicon-based optical modulatorusing a pn junction and operated with reverse bias [A. Liu, et al.,“High-speed optical modulation based on carrier depletion in a siliconwaveguide,” OPTICS EXPRESS, vol. 15, no. 2, pp. 660-668 (2007)]. T.Pinguet et al. have proposed a silicon-based optical modulatorcompatible with fabricating processes of CMOS devices [T. Pinguet, elal., “A 1550 nm, 10 Gbps optical modulator with integrated driver in 130nm CMOS,” Proc. of Group Four Photonics, ThA2, pp. 186-189 (2007)]. Bothof these optical modulators can operate at high speed.

FIG. 1 shows the sectional structure of the optical modulator proposedby Liu et al. FIG. 1 shows a cross section of the optical modulatortaken on a surface that is perpendicular to the direction in which lightpropagates. In the optical modulator, oxide layer 25 is formed on thetop surface of silicon substrate 24, and p-doped silicon layer 23 thatis an SOI layer is provided on oxide layer 25. In this case, p-dopedsilicon layer 23 is formed such that the sectional structure thereofincludes a projecting portion serving as a core of a rib opticalwaveguide and slab portions arranged on the respective opposite sides ofthe projecting portion and connected to the projecting portion. Toestablish an electric connection between p-doped silicon layer 23 andelectrode 27, p⁺⁺-doped silicon layer 22 to which a p-type dopant of ahigh concentration is introduced is provided so as to connect to each ofthe slab portions.

On p-doped silicon layer 23, n-doped silicon layer 21 is formed suchthat p-doped silicon layer 23 and n-doped silicon layer 21 form a pnjunction. A side portion of n-doped silicon layer 21 is connected ton⁺⁺-doped silicon layer 20 to which an n-type dopant of a highconcentration is introduced so as to be electrically connected toelectrode 28. A portion of n-doped silicon layer 21 which contactsp-doped silicon layer 23 also forms a part of the rib waveguide. Suchn-doped silicon layer 21 is formed by epitaxially growing a siliconlayer on the SOI layer (i.e., p-doped silicon layer 23) and dopingn-type impurities down to the vicinity of the interface between theepitaxially grown layer and the SOI layer.

Oxide layer 35 also functioning as a clad layer of the waveguide isprovided so as to entirely cover p-doped silicon layer 23, p⁺⁺-dopedsilicon layer 22, n-doped silicon layer 21, and n⁺⁺-doped silicon layer21.

In such an optical modulator, a reverse bias voltage is applied tobetween p-doped silicon layer 23 and n-doped silicon layer 21 viaelectrodes 27 and 28. Then, owing to the carrier plasma effect, amodulation operation is performed on light passing through p-dopedsilicon layer 23 and n-doped silicon layer 21, which form the ribwaveguide.

If the optical modulation portion has such a structure, the shape of theoptical modulation portion and the external electrode portion connectedto the optical modulation portion is different from the structure of thewaveguide connected to the electrodes. Thus, a connection structureneeds to be formed which optically couples the optical modulationportion and the external electrode portion to the waveguide structurewith a reduced loss and which enables high-speed electric responses.

Furthermore, in the structure shown in FIG. 1, light is confined in therib waveguide. However, if there is only a short distance from p⁺⁺-dopedsilicon layer 22 connected to the slab of the rib waveguide to the mainbody of the rib waveguide, optical absorption by the p⁺⁺-doped siliconmay disadvantageously result in a loss. Specifically, the p⁺⁺-dopedsilicon has a higher optical absorbance than p⁺⁺-doped silicon or puresilicon (or intrinsic silicon), which has a lower dopant concentration.Thus, the p⁺⁺-doped silicon cannot be used in the central portion of therib waveguide. Besides the central portion of the rib waveguide, thevicinity of the rib structure is not preferable as a position where thep⁺⁺-doped silicon is placed, in terms of a reduction of optical loss.

On the other hand, to allow the optical modulator to operate at highspeed, the electrodes are desirably arranged near the rib structure toreduce electric resistance associated with the electrodes. Hence,arranging the electrodes near the rib waveguide in order to increase thespeed of the electro-optic device is in a tradeoff relationship witharranging the electrodes away from the rib waveguide in order to reducean optical loss in the electro-optical device.

In the configuration in which electrode 27 is connected to p⁺⁺-dopedsilicon layer 22 as shown in FIG. 1, the following problem may occur ifthe thickness of p⁺⁺-doped silicon layer 22 is reduced in order toenhance confinement of light in the rib waveguide structure to increaseefficiency. That is, a manufacturing variation is likely to occur in thethickness of a silicide layer serving as a contact layer betweenelectrode 27 and p⁺⁺-doped silicon layer 22. This makes a contactresistance unstable.

In the optical modulator shown in FIG. 1, a pn junction is formed.However, WO2004/088394 discloses an example of a silicon-basedelectro-optic modulator in which a SIS(Semiconductor-Insulator-Semiconductor) junction is formed instead ofthe pn junction. This optical modulator has a waveguide structure inwhich a p-doped silicon layer and an n-doped silicon layer are stackedvia a relatively thin dielectric layer. When a modulation signal isapplied to between the two silicon layers, free carriers are accumulatedor removed or the carrier concentration is reversed, on the respectiveopposite sides of the dielectric layer. This changes an effectiveoptical refractive index.

FIG. 2 shows the sectional structure of the optical modulator proposedby Pinguet et al. FIG. 2 shows a cross section of the optical modulatortaken across a surface perpendicular to the direction in which lightpropagates. This optical modulator also includes a rib waveguide of anSOI structure but is different from the one shown in FIG. 1 in that ap-doped silicon layer and an n-doped silicon layer are arranged so as toform a lateral pn junction along a centerline extending in thelongitudinal direction of the rib waveguide such that the plane of thepn junction is perpendicular to the silicon substrate. Each of the dopedsilicon layers has a low impurity concentration in the rib waveguideportion. In the slab portion of the rib waveguide, the impurityconcentration increases with the distance from the rib waveguide.Pinguet et al. fail to fully describe the specific structure of theoptical modulator. However, p⁺-doped silicon layer 32 and n⁺-dopedsilicon layer 30 have larger thicknesses at each position in the slabportion where the slab portion contacts the electrode. In thisstructure, a stable silicide layer is expected to be formed so as toserve as a contact layer.

In the optical modulator or electro-optic device including the ribwaveguide of the SOI structure, it is completely unclear how close thecontact layer is to be located with respect to the rib waveguide for ahigh speed operation and how, in association with connection to theelectrode, the contact layer is to be located with respect to thecentral portion of the waveguide, through which light passes.Furthermore, silicon blocks provided on the slab portions on therespective opposite sides of the rib waveguide may cause a propagationloss or an insertion loss resulting from stray light.

SUMMARY OF THE INVENTION:

As one of silicon-based electro-optic devices that can be integrated ona silicon (Si) semiconductor substrate, the optical modulator based onthe carrier plasma effect can realize, in a very small size of submicronorder, a reduction in costs, reduction of current density, reduction ofpower consumption, a high modulation degree, driving at a reducedvoltage and high speed modulation. However, as described above, it isdifficult to achieve an efficient optical connection for such an opticalmodulator based on the carrier plasma effect by adjusting thearrangement of the electrode contact layer. Furthermore, a manner ofarranging the electrode contact layer which enables both the high-speedoperation and a reduction in optical propagation loss in the opticalmodulator is unknown.

An electrode connection structure which achieves an efficient opticalconnection and which enables both high-speed operations and a reductionin optical propagation loss has been demanded in a silicon-based opticalmodulator formed on an SOI substrate.

According to an exemplary aspect of the present invention, anelectro-optic device includes: a silicon body region doped to exhibit afirst conductivity type; a silicon gate region doped to exhibit a secondconductivity type, the silicon gate region being disposed at least inpart over the silicon body region to define a contiguous area betweenthe silicon body region and the silicon gate region; a dielectric layerdisposed in the contiguous area between the silicon body region and thesilicon gate region, the combination of the silicon body region and thesilicon gate region with the interposed dielectric layer defining anactive region of the electro-optic devices; a first electrical contactcoupled to the silicon gate region; and a second electrical contactcoupled to the silicon body region, wherein upon application of anelectrical signal to the first electrical contact and the secondelectrical contact, free carriers accumulate, deplete or invert withinthe silicon body region and the silicon gate region on both sides of thedielectric layer at the same time, such that an optical electric fieldof the optical signal substantially overlaps with a modulation area offree carrier concentration in the active region of the electro-opticdevice, a rib waveguide is formed by means of the silicon body region,the silicon gate region and the dielectric layer, a first electricalcontact is coupled to a slab part of the rib waveguide, the rib part hasa region coupled to an electrode contact region where thickness of theelectrode contact region of rib part is thicker than around rib part,and a position of the electrode contact region of the rib part islocated at a position of center of the waveguide where an effectiveindex value of the rib waveguide in a zeroth-order mode is constant whenthe distance between rib waveguide and the electrode contact regionvaries.

According to another exemplary aspect of the present invention, anelectro-optic device includes a first silicon layer of a firstconductivity type, a second silicon layer of a second conductivity typedifferent from the first conductivity type, the second silicon layerbeing at least partly stacked on the first silicon layer, a dielectriclayer formed at an interface between the first silicon layer and thesecond silicon layer, a first electric contact connected to the firstsilicon layer, and a second electric contact connected to the secondsilicon layer, wherein the first silicon layer, the dielectric layer,and the second silicon layer form an SIS junction, and an electricsignal from each of the first and second electric contacts allows a freecarrier to be accumulated, removed, or reversed on respective oppositesides of the dielectric layer to modulate a free-carrier concentrationsensed by an electric field of an optical signal, wherein a stackstructure including the first silicon layer and the second silicon layerhas a rib waveguide shape so as to form an optical confinement area, anda slab portion of the rib waveguide includes an area to which a metalelectrode is connected as the first electric contact, wherein the slabportion in the area to which the metal electrode is connected is thickerthan a surrounding slab portion, and wherein the area to which the metalelectrode is connected is located so that a range of a distance from therib waveguide to the area to which the metal electrode is connected issuch that when the distance is changed, an effective refractive index ofthe rib waveguide in a zeroth-order mode does not change.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description based onthe accompanying drawings which illustrate exemplary embodiments of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 is a sectional view showing an example of a structure of asilicon-based optical modulator with a pn junction;

FIG. 2 is a diagram showing an example of a connection structure for theoptical modulator and an electrode;

FIG. 3 is a diagram showing another example of the connection structurefor the optical modulator and the electrode;

FIG. 4 is a sectional view of the connection structure for the opticalmodulator and the electrode according to a first exemplary embodiment;

FIG. 5 is a sectional view showing an optical modulator according to asecond exemplary embodiment;

FIG. 6 is a graph showing the dependence of the optical modulator shownin FIG. 5 on the distance to an Si block in an optical waveguide mode;and

FIGS. 7A to 7F are sectional views sequentially showing a process offabricating the optical modulator.

DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

Before description of exemplary embodiments, a portion of a ribwaveguide where a slab is connected to an electrode structure will bedescribed.

To allow the refractive index of a silicon waveguide to be changed usingthe carrier plasma effect, an external voltage needs to be applied to acore portion of the waveguide. To achieve this, an electrode isconnected to a portion of the rib waveguide which extends from a slab.Metal such as aluminum (Al) or copper (Cu) is generally used as theelectrode. The portion extending from the slab and connected to theelectrode is configured with a silicon layer doped with impurities of ahigh concentration, so as to serve as a contact layer. The portion inwhich the contact layer and the metal electrode are connected togetheris configured with a silicide layer to allow the electrode and thecontact layer to be reliably electrically connected together.

FIG. 3 shows an electro-optic device which is similar to the deviceshown in FIG. 1 but which includes a contact layer, that is, p⁺⁺-dopedsilicon layer 22 with the thickness thereof reduced in order to decreasean optical propagation loss in the rib waveguide. In this configuration,since p⁺⁺-doped silicon layer 22 is very thin, when an attempt is madeto form an electric contact via the silicide layer for connection tometal electrode 27, the electrical characteristics of the connection maybe unstable. Specifically, even though an attempt is made to formsilicide layers using the same fabricating process, the thickness mayvary among the resultant silicide layers, resulting in an inconstantcontact resistance. This is a serious problem.

As a solution to the inconstant contact resistance, an example of aconfiguration of an electro-optic device according to a first exemplaryembodiment is shown in FIG. 4. The electro-optic device shown in FIG. 4is configured so as to serve as, for example, a silicon-based opticalmodulator.

In the optical modulator shown in FIG. 4, as in the optical modulatorshown in FIG. 3, p⁺⁺-doped silicon layer 22 connected to each slab ofthe rib waveguide is formed to be thin. However, unlike the opticalmodulator shown in FIG. 3, p⁺⁺-doped silicon layer 22 is formed to bethicker at a position corresponding to the connection to electrode 27.In other words, p++-doped silicon layer 22 is formed so as to have atrapezoidal cross-section near the connection to electrode 27, in theoptical modulator shown in FIG. 4.

Since p⁺⁺-doped silicon layer 22 serving as a contact layer is thickerat the connection to electrode 27, a silicide layer can be stably formedat a position corresponding to the interface between p⁺⁺-doped siliconlayer 22 and electrode 27. The results of the present inventors'experiments show that a stable contact resistance is obtained whenp⁺⁺-doped silicon layer 22 is thicker than the slab portion of the ribwaveguide by a range of between 50 nm and 100 nm.

Now, the silicon-based optical modulator shown in FIG. 4 will bedescribed in further detail.

As with the silicon-based optical modulator shown in FIG. 1 or FIG. 3,in the silicon-based optical modulator shown in FIG. 4, oxide layer 25is provided on silicon substrate 24, and p-doped silicon layer 23serving as an SOI layer is further provided on oxide layer 25 as thelower half of the rib waveguide by patterning. On the top surface ofp-doped silicon layer 23, n-doped silicon layer 21 is provided so as toform a pn junction with p-doped silicon layer 23. In addition, n⁺⁺-dopedsilicon layer 20 is connected to a side portion of n-doped silicon layer21. Electrode 28 is connected to n⁺⁺-doped silicon layer 20. Oxide layer35 also functioning as a clad layer of the rib waveguide is provided tocover all over p-doped silicon layer 23, p⁺⁺-doped silicon layer 22,n-doped silicon layer 21 and n⁺⁺-doped silicon layer 20.

On each of the opposite sides of the rib waveguide which is formed as aprojecting portion of the SOI layer, the SOI layer is formed so as tohave a thickness of at most 100 nm and to function as a slab layer.Provided on the slab layer is p⁺⁺-doped silicon layer 22 serving as acontact layer which is used for connection to electrode 27. Electrode 27is composed of, for example, aluminum or copper. In the area in whichp⁺⁺-doped silicon layer 22 is joined to electrode 27, p⁺⁺-doped siliconlayer 22 is formed to have a thickness larger than the sickness of theslab portion by at least 50 nm.

In this case, the thickness of p⁺⁺-doped silicon layer 22 in the area inwhich the p⁺⁺-doped silicon layer is joined to electrode 27 is set to bethe same as that of the SOI layer on which the rib waveguide has notbeen patterned yet. Then, processes used to fabricate an opticalmodulator is simplified. This reduces a variation in contact resistanceand contributes to reducing costs.

In this structure, the thin SOI layer which becomes the slab of the ribwaveguide is formed adjacent to p⁺⁺-doped silicon layer 22. The distancefrom the contact portion in which electrode 27 is joined to p⁺⁺-dopedsilicon layer 22 to the central axis of the rib waveguide is designedsuch that electrode 27 is electrically connected to the rib waveguide ata sufficiently low resistance. Furthermore, the effective refractiveindex in the zeroth-order mode of the rib-type optical waveguidestructure is designed so as to be independent of the distance from thecontact portion of electrode 27 to the central axis of the ribwaveguide. This arrangement of electrode 27 and the like allowsavoidance of the adverse effect of an optical absorption loss caused bythe electrode and the highly doped silicon region, and enables theelectrode to be designed so as to maintain an optical propagation mode.

FIG. 5 shows an electro-optic device according to a second exemplaryembodiment. To enable the electro-optic device to operate at high speed,an electrode for application of a modulation voltage is preferablyarranged in proximity to the modulation portion in which light interactswith carriers in the semiconductor. However, silicon doped withimpurities absorbs light, and the electrode arranged in proximity to themodulation portion leads to an increase in optical loss. Thus,realization of high-speed operations is in a tradeoff relationship witha reduction in optical loss. The second exemplary embodiment attempts tofind the optimum condition for the electrode arrangement in connectionwith the tradeoff relationship.

An electro-optic device shown in FIG. 5 is similar to that shown in FIG.4, but is different from that shown in FIG. 4 in that thin dielectriclayer 29 is formed at the interface between doped silicon layers 21 and23, instead of forming the pn junction at which n-doped silicon layer 21is connected directly to p-doped silicon layer 23 forming the ribwaveguide. That is, this electro-optic device includes an SIS junction.

In this electro-optic device, when an electric signal is applied tobetween p-doped silicon layer 23 and n-doped silicon layer 21 viaelectrodes 27 and 28, then, on the respective opposite sides ofdielectric layer 29, free carriers are accumulated or removed or thecarrier concentration is reversed. As a result, the concentration offree carriers, which acts on the electric field of an optical signalpropagating through the rib waveguide is modulated in accordance withthe electric signal. Hence, the effective optical refractive index ofthe waveguide is modulated. In this structure, p-doped silicon layer 23forms a silicon body area, and n-doped silicon layer 21 forms a gatearea. The silicon body area and the gate area, sandwiching thindielectric layer 29, form an active area of the electro-optic device.

Each projecting portion of p⁺⁺-doped silicon layer 22 to which electrode27 is joined extends along the longitudinal direction of the ribwaveguide to form a parasitic rib. Therefore, the electro-optic devicehas three silicon blocks A, B and C each extending along thelongitudinal direction. Silicon blocks A and C are the highly-dopedparasitic ribs each of which is made of p⁺⁺⁻doped silicon layer 22.Silicon block B is the center rib portion of the rib waveguide.

In FIG. 5, distance D indicates the distance from the contact portion inwhich electrode 27 is joined to p⁺⁺-doped silicon layer 22 to thecentral axis of the rib waveguide. In the present exemplary embodiment,distance D serves as a parameter that determines the tradeoffrelationship in the electro-optic device between the realization of thehigh-speed operation and a reduction in optical loss. Varying distance Dallows the characteristics of the modulation portion and thesurroundings thereof as an optical waveguide to be examined. The pointof the present exemplary embodiment is that the characteristics of thesurroundings as an optical waveguide are examined to determine theoptimum value for the electrode position in connection with the tradeoffrelationship.

FIG. 6 is a plot obtained by examining all the optical waveguide modesof silicon blocks in FIG. 5 with respect to varying distance D. In thegraph, the axis of ordinate indicates the effective refractive index.The axis of abscissa indicates distance D. Several sets each includingthree quadrangles, i.e., two rectangles A and C and one square B, areshown in the right side of the graph. The quadrangles A, B and Ccorrespond to the three silicon blocks A, B and C labeled in FIG. 5,respectively. A circle in the quadrangle indicates a propagation mode inwhich light propagates in the corresponding silicon block (the ribwaveguide or parasitic rib). Furthermore, in the graph, a mode in whicha modulation operation is to be performed is shown by a solid line. Theother modes are shown by dashed lines. Obviously, in the mode in which amodulation operation is to be performed, light propagates throughcentral silicon block B, that is, the rib waveguide.

The following is apparent from the graph in FIG. 6. In the mode shown bya solid line and which is to be used for modulation operation, that is,the mode in which light propagates through the rib waveguide, in aportion of the mode in which the effective refractive index variesdepending on distance D, the propagation mode is coupled to the otherpropagation modes shown by dashed lines. If the propagation mode iscoupled to the other propagation modes, the silicon block fails tooperate normally as an electro-optic device. Furthermore, even forregions in which curves each indicating the effective refractive indexfor the corresponding propagation mode depending on distance D crosseach other, it is desirable to avoid using some of the regions for whichdetermining whether or not the curves cross each other is difficult. Inthe present example, provided that distance D is at least about 0.6 umor more and the vicinity corresponding to distance D=0.7 μm is excluded,then it is possible to predict that coupling to other modes which mayresult in a loss is prevented, allowing the silicon blocks to operate asa normal waveguide. Thus, the results of the examination clearlyindicate where the electrode is to be located so as to lie as close tothe rib waveguide as possible with the arrangement of the silicon blocksnear the rib waveguide taken into account.

Also in the electro-optic device shown in FIG. 5, distance D is set suchthat electrode 27 is electrically connected to the rib waveguide at asufficiently low resistance and such that the effective refractive indexin the zeroth-order mode of the rib-type optical waveguide structure isset independently of distance D. This arrangement of electrode 27 andthe like allows avoidance of the adverse effect of an optical absorptionloss caused by the electrode and the highly doped silicon area, andenables the electrode to be designed so as to maintain the opticalpropagation mode.

Now, a process of fabricating the electro-optic device shown in FIG. 5,that is, the silicon-based optical modulator, will be described.

FIG. 7A shows the sectional configuration of an SOI substrate used toform an electro-optic device. The SOI substrate is configured such thatburied oxide layer 25 is provided on silicon substrate 24, with asilicon layer of thickness about 300 nm stacked on oxide layer 25. Toreduce an optical loss, the buried oxide layer preferably has athickness of at least 1,000 nm. A silicon layer on buried oxide layer 25is what is called an SOI layer and needs to be p-doped silicon layer 23.Here, assuming that the first and second conductivity types are definedas a p-type and an n-type, respectively, the silicon layer on oxidelayer 25 may be doped so as to exhibit the first conductivity type inadvance. Alternatively, the following process is possible. Afterformation of a silicon layer on buried oxide layer 25, phosphorous (P)or boron (B) is doped into the surface layer of the silicon layer by ionimplantation or the like. Thereafter, the resultant substrate issubjected to thermal treatment so as to diffuse p-type impuritiesthroughout the silicon layer, thus forming the silicon layer intop-doped silicon layer 23.

Then, as shown in FIG. 7B, a resist pattern is formed on the surface ofp-doped silicon layer 23, and a reactive etching process is applied tothe resist pattern to process p-doped silicon layer 23 into a ribwaveguide shape. Furthermore, an area with p-type dopant of a highconcentration doped therein is formed into p⁺⁺-doped silicon layer 22,which is a silicon block portion located adjacent to the rib waveguidestructure.

Then, as shown in FIG. 7C, oxide layer 35 used as a clad layer is formedby depositing an SiO₂ film of thickness 300 to 700 nm all over p-dopedsilicon layer 23 and p⁺⁺-doped silicon layer 22. Then, oxide layer 35 isflattened by a technique such as CMP (Chemical-Mechanical Polishingprocess) so as to control the height of the rib waveguide.

Next, as shown in FIG. 7D, a resist mask pattern is formed on thesurface of oxide layer 35 processed by CMP, and the reactive etchingprocess is applied to the resist mask pattern to form a trenchcorresponding to the position of the rib waveguide, in the oxide cladlayer. Then, p-doped silicon layer 23 serving as the core portion of therib waveguide is exposed from the bottom surface of the trench.

The mask layer used for the patterning is removed. Then, as shown inFIG. 7E, a silicon oxide layer that is relatively thin dielectric layer29 is formed on the SOI layer, that is, exposed p-doped silicon layer21, by thermal oxidation treatment. Dielectric layer 29 may be, insteadof the silicon oxide layer, for example, a silicon nitride layer, anyother high dielectric-constant (high-k) insulating layer, or a stack ofthese layers. Thereafter, a polycrystalline silicon layer is depositedto form n-doped silicon layer 21, and n⁺⁺-doped silicon layer 20 isformed in an electrode extracting portion of n-doped silicon layer 21.

Finally, as shown in FIG. 7F, oxide layer 35 as the clad layer isfurther deposited such that all the layers including n⁺⁺-doped siliconlayer 20 and n-doped silicon layer 21 are covered with the oxide cladlayer. Then, the surface of oxide layer 35 is flattened by CMP andcontact holes are formed such that electrodes 27 and 28 can be taken outthrough the contact holes. As a result, n⁺⁺-doped silicon layer 20 andp⁺⁺-doped silicon layer 22 are exposed from the bottom surfaces of thecontact holes. Thus, nickel (Ni) is deposited on the exposed portions ofn⁺⁺-doped silicon layer 20 and p⁺⁺-doped silicon layer 22 to form asilicide layer. Moreover, an electrode layer composed of TaN/Al (Cu) orthe like is formed in each of the contact holes to complete electrodes27 and 28. Electrodes 27 and 28 are used for connection to a drivingcircuit.

The whole or part of the exemplary embodiments disclosed above can bedescribed as, but not limited to, the following supplementary notes.

(Supplementary note 1) A silicon-based electro-optic device comprising:

a silicon body region doped to exhibit a first conductivity type;

a silicon gate region doped to exhibit a second conductivity type, thesilicon gate region being disposed at least in part over the siliconbody region to define a contiguous area between the silicon body regionand the silicon gate region;

a dielectric layer disposed in the contiguous area between the siliconbody region and the silicon gate region, the combination of the siliconbody region and the silicon gate region with the interposed dielectriclayer defining an active region of the electro-optic devices;

a first electrical contact coupled to the silicon gate region; and

a second electrical contact coupled to the silicon body region,

wherein upon application of an electrical signal to the first electricalcontact and the second electrical contact, free carriers accumulate,deplete or invert within the silicon body region and the silicon gateregion on both sides of the dielectric layer at the same time, such thatan optical electric field of the optical signal substantially overlapswith a modulation area of free carrier concentration in the activeregion of the electro-optic device,

a rib waveguide is formed by means of the silicon body region, thesilicon gate region and the dielectric layer,

a first electrical contact is coupled to a slab part of the ribwaveguide,

a rib part of the rib waveguide has a region coupled to an electrodecontact region where thickness of the electrode contact region of therib part is thicker than around the rib part, and

a position of the electrode contact region of the rib part is located ata position of center of the waveguide where an effective index value ofthe rib waveguide in a zeroth-order mode is constant when the distancebetween the rib waveguide and the electrode contact region varies.

(Supplementary note 2) The silicon-based electro-optic device as definedin Supplementary note 1, wherein a thickness of the electrode contactregion of the rib part is the same as a thickness of the rib waveguide.

(Supplementary note 3) The silicon-based electro-optic device as definedin Supplementary note 1, wherein a thickness of the electrode contactregion of said rib part is the same as a thickness at least in part ofan SOI region of the rib waveguide

(Supplementary note 4) The silicon-based electro-optic device as definedin Supplementary note 1, wherein a width of electrode contact region ofthe rib part is wider than a width of the rib waveguide.

(Supplementary note 5) An electro-optic device comprising:

a first silicon layer of a first conductivity type;

a second silicon layer of a second conductivity type different from thefirst conductivity type, the second silicon layer being at least partlystacked on the first silicon layer;

a dielectric layer formed at an interface between the first siliconlayer and the second silicon layer;

a first electric contact connected to the first silicon layer; and

a second electric contact connected to the second silicon layer,

wherein the first silicon layer, the dielectric layer, and the secondsilicon layer form an SIS junction, and an electric signal from each ofthe first and second electric contacts allows a free carrier to beaccumulated, removed, or reversed on respective opposite sides of thedielectric layer to modulate a free-carrier concentration sensed by anelectric field of an optical signal,

wherein a stack structure including the first silicon layer and thesecond silicon layer has a rib waveguide shape so as to form an opticalconfinement area, and a slab portion of the rib waveguide includes anarea to which a metal electrode is connected as the first electriccontact,

wherein the slab portion in the area to which the metal electrode isconnected is thicker than a surrounding slab portion, and

wherein the area to which the metal electrode is connected is located sothat a range of a distance from the rib waveguide to the area to whichthe metal electrode is connected is such that when the distance ischanged, an effective refractive index of the rib waveguide in azeroth-order mode does not change.

(Supplementary note 6) The electro-optic device as defined inSupplementary note 5, further comprising a silicon substrate and aburied oxide layer formed on the silicon substrate, wherein the firstsilicon layer is provided on the buried oxide layer so as to serve as anSOI layer.

(Supplementary note 7) The electro-optic device as defined inSupplementary note 5 or 6, wherein a slab thickness in the area to whichthe metal electrode is connected is the same as one of heights of thefirst silicon layer in the rib waveguide.

(Supplementary note 8) The electro-optic device as defined inSupplementary note 5 or 6, wherein a slab width in the area to which themetal electrode is connected is larger than the width of the firstsilicon layer in the rib waveguide.

(Supplementary note 9) The electro-optic device as defined inSupplementary note 5, wherein the slab thickness in the area to whichthe metal electrode is connected is at least 70 nm.

(Supplementary note 10) The electro-optic device as defined inSupplementary note 5, wherein the distance at which the effectiverefractive index does not change is at least one third of a wavelengthand at most two-thirds of the wavelength.

It will be apparent that other variations and modifications may be madeto the above described embodiments and functionality, with theattainment of some or all of their advantages.

It is an object of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

1. A silicon-based electro-optic device comprising: a silicon bodyregion doped to exhibit a first conductivity type; a silicon gate regiondoped to exhibit a second conductivity type, the silicon gate regionbeing disposed at least in part over the silicon body region to define acontiguous area between the silicon body region and the silicon gateregion; a dielectric layer disposed in the contiguous area between thesilicon body region and the silicon gate region, the combination of thesilicon body region and the silicon gate region with the interposeddielectric layer defining an active region of the electro-optic devices;a first electrical contact coupled to the silicon gate region; and asecond electrical contact coupled to the silicon body region, whereinupon application of an electrical signal to the first electrical contactand the second electrical contact, free carriers accumulate, deplete orinvert within the silicon body region and the silicon gate region onboth sides of the dielectric layer at the same time, such that anoptical electric field of the optical signal substantially overlaps witha modulation area of free carrier concentration in the active region ofthe electro-optic device, a rib waveguide is formed by means of thesilicon body region, the silicon gate region and the dielectric layer, afirst electrical contact is coupled to a slab part of the rib waveguide,the rib part has a region coupled to an electrode contact region wherethickness of the electrode contact region of rib part is thicker thanaround rib part, and a position of the electrode contact region of therib part is located at a position of center of the waveguide where aneffective index value of the rib waveguide in a zeroth-order mode isconstant when the distance between rib waveguide and the electrodecontact region varies.
 2. The silicon-based electro-optic device asdefined in claim 1, wherein a thickness of the electrode contact regionof the rib part is the same as a thickness of the rib waveguide.
 3. Thesilicon-based electro-optic device as defined in claim 1, wherein athickness of the electrode contact region of said rib part is the sameas a thickness at least in part of an SOI region of the rib waveguide 4.The silicon-based electro-optic device as defined in claim 1, wherein awidth of electrode contact region of the rib part is wider than a widthof the rib waveguide.